Superplastic behaviour in a number of aluminium alloys is known. It is generally required that the alloy should have a fine, stable, grain size (1 to 10 microns) or be capable of achieving such a grain size during hot deformation; be deformable at a temperature not less than 0.7 Tm (melting temperature) and at strain rates in the range 10.sup.-2 to 10.sup.-5 sec.sup.-1.
In this specification where four figure numbers are used to specify aluminium alloys those are as designated by the Aluminum Association Inc.
It has been found that the two most important routes to achieve superplasticity are as follows:
(1) With alloys which have a composition suitable for superplastic deformation but a grain structure which precludes it. With such alloys the grain structure can frequently be modified by an initial non-superplastic deformation step at a suitable forming temperature to induce dynamic recrystallisation so that a fine recrystallised grain structure is progressively developed and superplastic deformation can then take place. Such alloys may for example include 2004 and its derivatives and the process is described in UK Patent 1456050. PA1 (2) with alloys such as 7075 and 7475 that are subjected to a static recrystallisation treatment as their final stage in complex thermomechanical processing to develop a fine, stable, grain structure. Such alloys are then inherently capable of subsequent superplastic deformation. Reference is made to work done by Rockwell International and to the publications "Superplasticity in High Strength Aluminium Alloys" pp. 173 to 189 and "Superplastic Forming of Structural Alloys", AIME New York 1982 (ISBN O-89520-389-8). PA1 Hot rolled product PA1 Heavy cold deformation PA1 Dynamic recrystallisation PA1 Superplastic deformation PA1 Hot rolled product PA1 Solution treatment PA1 Overageing process PA1 Cold or Warm deformation PA1 Static recrystallisation PA1 Superplastic deformation PA1 (1) holding the previously hot-rolled blank at a temperature between 275.degree. C. and 425.degree. C. for between 1 and 24 hours PA1 (2) allowing the blank to cool to a temperature suitable for cold forming PA1 (3) cold forming the blank in at least two stages and PA1 (4) annealing the cold formed blank between each of said stages at a temperature of between 300.degree. C. and 400.degree. C. for no more than 2 hours using a controlled heat-up rate of between 10.degree. C. and 200.degree. C./hour and allowing the annealed product to cool.
Aluminium/lithium alloys such as 8090 and 8091 appear to possess many of the characteristics of the 2004 type in that they can be made to develop a fine grain structure by dynamic recrystallisation from an original grain structure not suitable for superplastic deformation. (see R. Grimes and W. S. Miller in "Aluminium-Lithium 2, Monterey, Calif. 1984"). We have also shown, in UK Patent 2,139,536 how superplastic deformation of an Al/Li alloy can be achieved by modifying its high temperature deformation characteristics.
More recently it has been shown (J. Wadsworth, C. A. Henshall and T. G. Nieh "Superplastic Aluminium-Lithium alloys" in Aluminium Lithium Alloys 3 ed. C. Baker, P. J. Gregson, S. J. Harris and C. J. Peel, Pub. Inst of Metals 1986 p 199) that this type of processing route can also be applied to a variety of aluminium-lithium based alloys to create a superplastically deformable grain structure.
Aluminium/lithium alloys are therefore unusual in that both processing routes can be applied to the same starting alloy chemistry to achieve superplasticity. Work by Wadsworth et al (see above) has shown that good superplastic performance can be achieved by either process route.
Thus the two most important superplastic deformation routes, as discussed above, can be summarised as follows.
Route 1 (corresponding with paragraph numbered 1 above)
In the case of 2004 and its derivatives it is essential, for Route 1, to cast the ingot in such a way that it is supersaturated with zirconium.
Route 2 (corresponding with paragraph numbered 2 above)
It must be emphasised that these two routes have been developed separately in respect of different types of alloys. Apart from each starting from a hot rolled product and ending in a superplastic deformation step they differ considerably in conformity with the differing properties of the alloys to which they have been applied.
In many aluminium base alloys grain control constituents such as zirconium are included and when the Zr content increases above about 0.15% casting to produce a good product becomes progressively (and considerably) more difficult.